If a coat colour is not on the MBL I suggest you doing some research and get evidences that that coat colour/marking exists on the breed. If you register the horse first it will be declined and will ask you to change the coat colour or breed. ^^

Maybe because the breed is very restrictive, for example Menorquin horses can only be black. Although you can always suggest adding new coat colours, markings to the breed you talk about with real evidence of course. Just curious, what's the breed ?